henry lj et al 2010 'enhancement of output power from narrow linewidth amplifiers' optics...

Enhancement of output power from narrow

linewidth amplifiers via two-tone effect - high

power experimental results

Leanne J. Henry,1,*

Thomas M. Shay,1 Dane W. Hult,

2 and Ken B. Rowland Jr.

3

1Air Force Research Laboratory, Directed Energy Directorate, 3550 Aberdeen Avenue SE,

Kirtland Air Force Base, New Mexico 87117, USA 2TREX Enterprises Corporation, 10455 Pacific Center Court, San Diego, California 92121, USA

3Boeing LTS Inc., P.O. Box 5670, Albuquerque, New Mexico 87185, USA

*[email protected]

Abstract: Two-tone 1064 nm fiber amplifiers having both cold (16°C) and

pump induced temperature zones co-seeded with narrow linewidth 1064 nm

and broad linewidth 1040 nm photons have been shown to have a power

enhancement factor between 1.6 and 1.8 relative to the optimum single-tone

1064 nm amplifier while maintaining an efficiency of 65% or greater. The

output power and efficiency of 1064 nm narrow linewidth two-tone

amplifiers is dependent on the length of the gain fiber, the narrow to broad

linewidth seed ratio, the wavelength of the broad linewidth seed and the

temperature of the gain fiber.

©2010 Optical Society of America

OCIS codes: (060.2320) Fiber optics and optical communications; (140.3510) Lasers and laser

optics.

References and links

1. J. P. Koplow, D. A. Kliner, and L. Goldberg, “Single-mode operation of a coiled multimode fiber amplifier,”

Opt. Lett. 25(7), 442–444 (2000).

2. D. P. Machewirth, Q. Wang, B. Samson, K. Tankala, M. O'Connor, and M. Alam, “Current developments in

high-power, monolithic, polarization maintaining fiber amplifiers for coherent beam combining applications,”

Proc. SPIE 6453, 64531F (2007).

3. Y. Jeong, J. Nilsson, J. K. Sahu, D. N. Payne, R. Horley, L. M. B. Hickey, and P. W. Turner, “Power scaling of

single frequency ytterbium-doped fiber master-oscillator power-amplifier sources up to 500 W,” IEEE J. Sel.

Top. Quantum Electron. 13(3), 546–551 (2007).

4. A. Wada, T. Nozawa, D. Tanaka, and R. Yamauchi, “Suppression of SBS by intentionally induced periodic

residual-strain in single-mode optical fibers,” in Proceedings of the 17th ECOC, 1991, 25–28.

5. M. J. Li, X. Chen, J. Wang, S. Gray, A. Liu, J. A. Demeritt, A. B. Ruffin, A. M. Crowley, D. T. Walton, and L.

A. Zenteno, “Al/Ge co-doped large mode area fiber with high SBS threshold,” Opt. Express 15(13), 8290–8299

(2007).

6. M. D. Mermelstein, M. J. Andrejco, J. Fini, C. Headley, and D. J. DiGiovanni, “11.2 dB SBS gain suppression in

a large mode area Yb-doped optical fiber,” Proc. SPIE 6873, 68730N (2008).

7. B. Shiner, “Recent technical and marketing developments in high power fiber lasers,” in Tech Focus: Fiber

Lasers and Amplifiers: Concepts to Applications, CLEO Europe, Munich, Germany, 2009.

8. T. Bronder, I. Dajani, C. Zeringue, and T. Shay, “Multi-tone driven high-power narrow linewidth rare earth

doped fiber amplifier,” US Patent 7764720.

9. I. Dajani, C. Zeringue, T. J. Bronder, T. Shay, A. Gavrielides, and C. Robin, “A theoretical treatment of two

approaches to SBS mitigation with two-tone amplification,” Opt. Express 16(18), 14233–14247 (2008).

10. I. Dajani, C. Zeringue, and T. M. Shay, “Investigation of nonlinear effects in multitone-driven narrow-linewidth

high-power amplifiers,” IEEE J. Sel. Top. Quantum Electron. 15(2), 406–414 (2009).

11. C. Lu, I. Dajani, C. Zeringue, C. Vergien, L. Henry, A. Lobad, and T. Shay, “SBS suppression through seeding

with narrow linewidth and broadband signals: experimental results,” Proc. SPIE 7580, 75802L (2010).

1. Introduction

A high power laser system having a diffraction limited beam is desirable for long range

strategic applications. To simplify such a system composed of multiple fiber amplifier legs, it

is necessary to increase the output power of the individual fiber legs. For narrow linewidth

#134378 - $15.00 USD Received 31 Aug 2010; revised 15 Oct 2010; accepted 15 Oct 2010; published 29 Oct 2010(C) 2010 OSA 8 November 2010 / Vol. 18, No. 23 / OPTICS EXPRESS 23939

amplifiers, the major impediment to accomplishing this is Stimulated Brillouin Scattering

(SBS). Numerous methods have been suggested or implemented to mitigate SBS to include

large-mode area fibers [1], thermal gradients [2,3], stress [4], along with various

manipulations of the acoustic properties of the core or cladding regions of the gain fiber [5,6].

A promising method to suppress SBS in the long wavelength gain region involves the usage

of a tandem pumping two-stage brightness enhancing technique [7]. Specifically, a

multiplicity of 976 nm diodes are first converted to several 1010 nm single mode lasers which

are then combined to pump a low area ratio double clad fiber for amplification of the 1070 nm

signal. The result is a reduced nonlinear length relative to that obtained when the 1070 nm

amplifier is pumped directly with 976 nm diodes. This technique has enabled SBS free

amplifiers in the 1 kW regime and SRS free amplifiers at 10 kW. Another method to suppress

SBS is a two-tone technique where a narrow linewidth fiber amplifier is co-seeded with a

broad linewidth seed [8]. In a co-pumped configuration under proper selection of wavelengths

and seed power ratio, it has been shown theoretically that the SBS process can be mitigated

and an increased output power can be obtained as a result of power transfer from the broad to

the narrow linewidth tones due to laser action [9,10]. Ideally, it is desirable for the power

transfer to be completed by the end of the gain fiber. For the two-tone amplifier system (976

nm pump, 1040 nm broad linewidth seed, and 1064 narrow linewidth seed) studied in this

paper, ideal intensity profiles enacting complete power transfer would appear as shown in Fig.

1. Utilization of a broad linewidth co-seed avoids the possibility of reaching the SBS

threshold prematurely.

As seen in Fig. 1 below, the intensity of 1064 nm in the gain fiber tends to mimic that

found for counter-pumped amplifiers. An amplifier having a 1064 nm intensity distribution as

shown in Fig. 1 will have a shorter effective length for SBS and will therefore have higher

output power. By two-tone seeding, the benefits of counter-pumping may be achieved using

the readily available monolithic and proven components for the co-pumping system. Recently,

two-tone seeding was investigated at lower powers by co-seeding an amplifier having 10 m of

10/125 PM Yb-doped double clad gain fiber with narrow linewidth 1064 nm and broad

linewidth 1045 nm radiation [11]. An increase in SBS threshold was found for the two 1045

nm/1064 nm seed ratios of 10.8 and 5.4. The increase in SBS threshold could not be

quantified because the experiment was pump limited at approximately 8.5 W of 976 nm

radiation. In this work, the results of experiments performed at higher powers are presented.

2. Investigation of two-tone amplifier trade space

The performance enhancement of two-tone amplifiers relative to single-tone amplifiers

depends on the length of the gain fiber, the 1040 nm/1064 nm seed ratio, and the temperature

distribution in the gain fiber. The parameter space associated with two-tone amplifiers was

explored as fully as possible for lengths of gain fiber between 1 and 10 m. The gain fiber in

both the two-tone and the comparison single-tone amplifiers was spooled in two temperature

zones with the temperature zone furthest from the end of the fiber at 16°C and the temperature

zone closest to the end of the fiber at a pump induced temperature. Two-tone


Fig. 1. Ideal intensity profiles in a 7 m gain fiber showing a complete power transfer from the

1040 nm to the 1064 nm signal by the end of the gain fiber.

amplifiers co-seeded with narrow linewidth 1064 nm and broad linewidth 1040 nm radiation

were compared with the optimum single-tone 1064 nm amplifier to determine the power

enhancement factor and the optimum operating point.

2.1 Experimental methodology

A high power fiber amplifier pumped with wavelength stabilized 976 nm radiation was co-

seeded with narrow linewidth 1064 nm and broad linewidth 1040 nm signals. A schematic of

the two-tone system is shown in Fig. 2. The two seeds were first amplified to a maximum of

3-4 W prior to entering the wavelength division multiplexer (WDM). The combined power of

the two seeds was limited by the WDM which was rated at 3 W. From high resolution spectra,

both seed sources were found to be spectrally pure with amplified spontaneous emission

power less than 40 dB below the signal and not measurable. From the WDM, the two seeds

were injected into the high power amplifier and eventually into Nufern generation 7 PM

25/400 Yb-doped double-clad gain fiber. The amplifier was measured to have a beam quality

given in terms of M2 of 1.1.

The same amplifier was utilized for both single- and two-tone experiments with the

difference being the number of seeds injected into the WDM. For both single- and two-tone

amplifiers, the unabsorbed pump was removed from the amplifier output via a dichroic, M1.

For two-tone amplifiers, 1040 nm in the output was removed via a spike filter, M2, which

transmitted 1064 nm and reflected virtually all other wavelengths emitted by the high power

Yb amplifier under test. For single-tone amplifiers, the following quantities were recorded

for

1064 nm source

1040 nm source

Zone 1

(16C)

Zone 2

(pump induced)

3W WDM

Gain fiber

976 nm pumps

Fig. 2. Diagram of two-tone experimental set-up.

each pump level: unabsorbed pump power, 1064 nm output power, and backward tap coupler

power. For two-tone amplifiers, 1040 nm output power was recorded in addition to the

quantities listed above, see Fig. 3.


Fig. 3. Output end of two-tone amplifier showing measurement of the following in the output

of the amplifier: unabsorbed pump, 1040 nm and 1064 nm. A dichroic (M1) was used to

remove the unabsorbed pump from the beam and a spike filter (M2) was used to separate the

1040 nm from the 1064 nm.

In order to further enhance the suppression of SBS, the gain fiber for both single- and two-

tone amplifiers was placed in two temperature zones. The temperature zone furthest from the

output end of the gain fiber was cooled to a temperature of approximately 16°C. The

temperature zone closest to the output end of the gain fiber was allowed develop a pump

induced thermal gradient. For both single- and two-tone amplifiers, the gain fiber was placed

in the two temperature zones in such a way so as to equalize the SBS gain in each temperature

zone, thereby maximizing the output power that would be achieved before SBS occurred. For

single-tone amplifiers, it was found that placing 57.5% of the gain fiber in the cold zone and

42.5% in the zone of pump induced temperature gradient resulted in equal SBS in the two

temperature zones. For two-tone amplifiers, the intensity of 1064 nm along the length of gain

fiber is different than that for single-tone amplifiers with much greater intensities occurring

toward the end of the gain fiber, see Fig. 1. In order to equalize the SBS gain in each

temperature zone for this case, approximately 82% of the gain fiber must be placed in the cold

zone with the remainder, roughly 18%, in the pump induced temperature zone.

To determine the set of optimum operating parameters for two-tone amplifiers, various

lengths of gain fiber as well as 1064 and 1040 nm seed levels were explored. Two-tone

amplifiers having lengths of gain fiber between 1 and 10 m were seeded with 1064 nm seed

powers that varied between 39.5 and 236 mW and with 1040 nm seed powers that varied

between 295 to 2456 mW. Only seed combinations resulting in amplifiers having less than

20% 1040 nm in the output or of roughly 60% or greater efficiency were investigated. Each

amplifier was run up in power until the SBS threshold was reached. This was done by

monitoring the power out of the backward tap coupler. At the SBS threshold, which was

defined to be the point where the SBS peak was approximately 20 dB above the Rayleigh

peak, the maximum power from the amplifier was recorded. Comparison single-tone

experiments, seeded with approximately 2.3 W of 1064 nm, were performed at each length of

gain fiber as well.

2.2 Experimental results

2.2.1 Profile of 1040 and 1064 nm along the gain fiber

The physical processes responsible the power enhancement found in two-tone amplifiers can

be illustrated by examining the power profile of the 1040 and 1064 nm along the length of the

gain fiber for two distinct cases. The two-tone amplifier represented in Figs. 4a-4b is seeded


(a)

(d)(c)

(b)

Fig. 4. a-d. Power profile for 1040 and 1064 nm as a function of gain fiber length for a seed

level of 1040 nm of 1.4 W and (a)/(b) a low 1064 nm seed level of 42.5 mW and (c)/(d) a high

1064 nm seed level of 785 mW.

with 42.5 mW of 1064 nm and 1436 mW of 1040 nm. It is representative of two-tone

amplifiers that are lightly seeded in 1064 nm and heavily seeded in 1040 nm (high 1040 nm /

1064 nm seed ratio). As one can see, the power profile of 1064 nm is initially suppressed by

growth of 1040 nm in the initial segment of the gain fiber. Approximately 5 m down the gain

fiber, power transfer from the 1040 nm to the 1064 nm starts to accelerate with power transfer

being nearly complete 10 m down the gain fiber. At lengths of gain fiber less than 10 m,

power transfer is not complete resulting in an increasing amount of 1040 nm in the output of

the amplifier for shorter lengths of gain fiber. The two-tone amplifier represented in Figs. 4c-

4d is seeded with 785 mW of 1064 nm and 1436 mW of 1040 nm (lower 1040 nm / 1064 nm

seed ratio). It is representative of two-tone amplifiers that are more heavily seeded in 1064

nm. The power profiles shown in Figs. 4c and 4d are different than those in Figs. 4a and 4b

primarily because the 1064 nm is competing on a more favorable basis with the 1040 nm for

the gain because of the increased seed level. The net result is that the 1064 nm tends to be less

suppressed in the initial segment of the gain fiber with the power transfer between the 1040

and 1064 nm accelerating 3 m down the gain fiber. With respect to amplifier performance, the

output power of the amplifier with a lower 1040 nm/1064 nm seed ratio will be less since the

effective length for SBS is greater resulting in a lowered SBS threshold. In addition, at all

lengths of gain fiber, the efficiency of such an amplifier will be greater since the output will

contain less 1040 nm.

2.2.2 Effect of co-seeding with 1040 nm

The effect of the level of 1040 nm seeding (or the size of the 1040 nm/1064 nm seed ratio) on

the SBS threshold or the power enhancement of two-tone amplifiers was investigated. Trends

associated with the level of 1064 and 1040 nm in the amplifier output along with the 1064 nm

amplifier efficiency are shown as a function of the 1040 nm seed level (from 0 to 2.45 W) for

a fixed level of 1064 nm seed in Figs. 5a and 5b. Figure 5a shows measurements associated


with a 7 m amplifier seeded with 0.485 W of 1064 nm and Fig. 5b shows measurements

associated with a 9 m amplifier seeded with 0.065 W of 1064 nm. The following trends are

associated with both amplifiers as the seed level of 1040 nm increases: the level of 1040 nm

in the output of the amplifier increases, the output power of 1064 nm increases, and the

efficiency of the 1064 nm amplifier decreases. The increase in the 1064 nm output power

occurs because the 1040 nm in the gain fiber is able to more effectively suppress the intensity

profile of 1064 nm in the gain fiber as the 1040 nm seed level increases resulting in a

decrease in the effective length for SBS (Fig. 4a versus Fig. 4c). But, as the 1040 nm is

seeded more heavily, power transfer to the 1064 nm becomes less and less complete resulting

in a greater amount of 1040 nm in the output of the amplifier with a subsequent decrease in

1064 nm efficiency, Fig. 4b versus Fig. 4d. Because the amplifier shown in Fig. 5a is seeded a

factor of 7.5 more 1064 nm than is the amplifier shown in Fig. 5b, the increase in 1040 nm

doesn't have as great an effect since the 1064 nm continues to compete favorably for the gain.

That is, the efficiency decreases only slightly from 76% to 72% and the amount of 1040 nm

in the output of the amplifier increases to approximately 10% at the highest 1040 nm seed

level. For the more lightly seeded amplifier shown in Fig. 5b, the amount of 1040 nm in the

output of the amplifier increases to approximately 40 W and the efficiency of the amplifier

drops off from roughly 82% to 55% at the highest seed level due to an inability of the 1064

nm to compete favorably for gain. What is also clear from both cases is that as the seed level

of 1064 nm increases relative to the 1040 nm seed (or the 1040 nm / 1064 nm seed ratio

decreases), the 1064 nm output power falls and the 1064 nm efficiency increases since the

power transfer from 1040 nm to 1064 nm occurs in a shorter length of gain fiber, Fig. 4d

versus Fig. 4b. As a result, the 1040 nm is unable to suppress the 1064 nm intensity profile in

the gain fiber as effectively (Fig. 4c versus Fig. 4a) resulting in an increase in effective length

for SBS and a decrease in the SBS threshold.

(a) (b)

Fig. 5. a and b. Power enhancement and efficiency of two-tone amplifiers for constant 1064

nm and variable 1040 nm seeds: (a) 7 m fiber amplifier co-seeded with 0.485 W of 1064 nm

along with up to 2.45 W of 1040 nm, (b) 9 m fiber amplifier co-seeded with 0.065W of 1064

nm along with up to 2.45 W of 1040 nm.

2.2.3 Optimum operating point for two-tone 1064 nm amplifier

The efficiency and output power of a 1064 nm two-tone amplifier is dependent on the seed

levels for 1040 and 1064 nm as well as the length of the gain fiber for the experimental

configuration described in this paper. Shown in Table 1 are the best two-tone amplifiers as a

function of the gain fiber length within each efficiency range. Each amplifier was driven to

the SBS threshold.


Table 1. Best two-tone 1064 nm amplifier in terms of power as a function of efficiency

and length of gain fiber

Efficiency ranges for 1064 nm amplifier

75-80% 70-75% 65-70% 60-65%

Gain fiber

length [m]

Maximum 1064 nm output power [W]

10 27.8 45.3 49.3 45.5

9 43.6 49.6 48.1 58.6

8 47.4 71.7 78.5 No data

7 77.5 79.8 84.4 80.2

6 68.8 74.7 75.5 76.8

5 Doesn't exist 62.8 76.5 88.8

Upon examination of Table 1, a surprising trend for the experimental configuration

considered is that for the higher 1064 nm amplifier efficiencies, the optimum length of gain

fiber (for maximum 1064 nm power) appears to 7 m versus a length of 5 m which is optimum

for single-tone amplifiers. This can be explained by the fact that the optimum 1064 nm/1040

nm seed ratio needs to be higher at the shorter lengths of gain fiber (5 and 6 m) in order to

keep the amount of 1040 nm in the output low and the efficiency high, see Table 2. This is the

case since the length of gain fiber required for an efficient transfer of power from the 1040

nm to the 1064 nm decreases as the seed ratio 1064 nm/1040 nm increases. This is shown

clearly in Figs. 4b and 4d. The net result is a decrease in suppression of the 1064 nm (see

Figs. 4a and 4c), an increase in the effective length for SBS, and finally, a decrease in the

1064 nm SBS limited output power. Another trend in Table 1 that was shown before is that

the output power of the 1064 nm tends to increase with decreasing efficiency. This is caused

by a decrease in the effective length for SBS due to improved suppression of 1064 nm in the

gain fiber when higher seed levels of 1040 nm are used. Finally, the best 1064 nm output

power achieved from the two-tone amplifiers investigated was 88.8 W at an efficiency of

64.6% out a 5 m fiber amplifier seeded with 0.3 W of 1040 nm and 0.485 W of 1064 nm.

Table 2 shows the power enhancement achievable for two-tone amplifiers with

efficiencies 70% at 1064 nm for lengths of gain fiber between 5 and 10 m. Upon

examination of Table 2, it is apparent, that the best power enhancement factor, 1.6, for a two-

tone amplifier relative to the optimum 70% efficient 5 m single-tone amplifier occurs when

the gain fiber has a length of 7 m. (A power enhancement factor of 1.8 was found for the 5 m

amplifier having an efficiency of 64.4% described above.) Relative to single-tone amplifiers

at the same length of gain fiber, a maximum power enhancement factor of 2.3 was found to

occur at lengths of 7 and 8 m.

Table 2. Enhancement of two-tone seeding relative to single-tone seeding at 1064 nm for

the optimum 70% efficient amplifier at each length.

Length

[m]

Maximum

power

(single-tone)

[W]

Maximum

power (two-

tone) [W]

Two-tone

efficiency

[%]

Enhancement

relative to

single-tone at

same length

Enhancement

relative to

optimum single-

tone at 5 m

Optimum ratio

P1064/P1040

5 50 62.8 70.3 1.26 1.26 4.19

6 45 68.8 76 1.53 1.38 1.64

7 35.5 79.8 71.3 2.25 1.60 0.20 to 0.60

8 31.3 71.7 71.6 2.29 1.43 0.10

9 27 49.6 71.8 1.84 0.99 0.10

10 25 45.3 73.3 1.81 0.91 0.10


2.3 Investigation of the effect of temperature of the second heating zone on efficiency and

output power

An investigation was carried out on three different 7 m two-tone amplifiers having 1064 nm

efficiencies in excess of 70% to see the effect of keeping the entire gain fiber cold versus

having two temperature zones as described above. The seed levels of the amplifiers involved

are shown in Table 3. When the entire gain fiber was kept cold, Fig. 6a, it was found that the

output power achievable prior to reaching the SBS threshold was significantly less than when

the last 18% or so of the gain fiber was allowed to achieve a pump induced temperature

gradient, Fig. 6b.

The output power was found to decrease, for this case, between 18 and 37.7% as the level

of the 1064 nm seed increased. This is as expected since the contribution to SBS when the

gain fiber is kept at one temperature occurs at one frequency versus two resulting in the SBS

threshold being reached at lower 1064 nm output powers. More surprising was the fact that

1064 nm source

1040 nm source

Zone 1

(16C)

Zone 2

(pump induced)

3W WDM

Gain fiber

976 nm pumps

1064 nm source

1040 nm source

Zone 1

(16C)

3W WDM

Gain fiber

976 nm pumps

VS

(a) (b)

Fig. 6. a b. Experimental setup utilized for comparing the effect of having: (a) the entire gain

fiber cold versus (b) having the gain fiber in two temperature zones.

the efficiency of the 1064 nm amplifier was also found decrease between 7.6 and 19.8 percent

when the entire gain fiber was kept cold. The decrease in 1064 nm efficiency appeared to be

greatest for amplifiers seeded heaviest with 1064 nm. This may be due to temperature

dependence of Yb absorption and emission cross-sections. Further investigation of this will

be the subject of another paper.

Table 3. Comparison of one versus two temperature regions for two-tone seeding in a 7 m

gain fiber

P1064

nm

[W]

seed

P1040

nm

[W]

seed

Ratio Pout

[1064

nm]

[W]

all

cold

Pout

[1064

nm] [W]

cold/hot

region

Percentage

decrease in

output

power [%]

Efficiency -

cold region

Efficiency -

cold/hot

region

Percentage

decrease in

efficiency

[%]

0.706 1.2 0.59 48.3 77.5 37.7 61.5 76.7 19.8

0.485 1.987 0.24 61.5 77.8 21 63.8 71.5 10.8

0.15 0.295 0.51 65.5 79.8 18 65.9 71.3 7.6

3. Discussion

For all cases, in two-tone amplifiers, when the 1040 nm seed level is increased for a fixed

seed level of 1064 nm a significant increase in the SBS threshold is observed due to a

decrease in the effective length for SBS. A potential downside of such amplifiers is lower

1064 nm efficiency since power transfer from the 1040 nm to the 1064 nm is typically not

complete due to the greater length of gain fiber required. In this case, the SBS limited power

increases but the amplifier efficiency at 1064-nm decreases. Correspondingly, in two-tone

amplifiers, when the 1064 nm seed level is increased for a fixed 1040 nm seed level, typically

a more complete power transfer from 1040 nm to 1064 nm occurs. The net result in the later

case is an increase in the effective length for SBS which results in a decreased SBS threshold.

Thus, we have a performance trade off the SBS limited power decreases but the 1064 nm

amplifier efficiency increases. Finally, the properties of a 1064 nm two-tone amplifier are

ultimately determined by the profiles of the 1040 and 1064 nm signals within the gain fiber.

Two-tone amplifiers comprised of shorter lengths of gain fiber (5 and 6 m) are particularly

plagued by an incomplete power transfer from 1040 nm to 1064 nm leading to a significant


amount of 1040 nm in the output along with decreased 1064 nm amplifier efficiency. The

percentage of 1040 nm in the output of the amplifier can be decreased at all lengths by

increasing the seed ratio of 1064 nm/1040 nm which has the net effect of shifting the point

where the power transfer is accelerated to a shorter length of gain fiber. It is also possible to

adjust the longitudinal power profiles of the 1040 and 1064 nm in the gain fiber by changing

the wavelength of the broadband seed because of the spectral dependence of the absorption

and emission cross-sections. An increase in the absorption cross-section or emission cross-

section of the broadband seed decreases and shifts the point of power transfer from 1040 to

1064 nm to an earlier position in the gain fiber. This has the effect of increasing the effective

length of SBS which in turn decreases the SBS limited 1064 nm output power and increases

the 1064 nm amplifier efficiency. The converse is true for a decrease in the absorption or

emission cross-sections. Any change to the wavelength of the broadband seed needs to be

done carefully since the absorption and emission cross-sections of Yb have different

dependencies on wavelength. For example, a broadband wavelength of 1050 nm would shift

the point of power transfer to 1064 nm to a later position in the fiber whereas a broadband

wavelength of 1028 nm would shift the point of power transfer to a an earlier position in the

fiber. Finally, it may also be possible to control the percentage of 1040 nm in the amplifier

output by adjusting the temperature of the end of the gain fiber. This latter idea will be

investigated in future work.

In summary, for the two-tone amplifier configuration described above with the gain fiber

in two temperature zones, the first at 16°C and the second at a pump-induced temperature, the

optimum length of the gain fiber appears to be 7 m for a minimum 70% efficient 1064 nm

amplifier. At a gain fiber length of 7 m, a maximum output power on the order of 78-80 W

can be achieved with a resulting power enhancement factor of 1.6 relative to the optimum

single-tone amplifier which occurs at a gain fiber length of 5 m. Higher output powers are

achievable from a shorter gain fiber, i.e., on the order of 90 W from a 5 m gain fiber, resulting

in a power enhancement factor of 1.8 but with decreased efficiency of 64.4% due to

incomplete power transfer from 1040 nm to 1064-nm at the amplifier output.

Acknowledgements

The authors would like to acknowledge Dr. Iyad Dajani and Mr. Clint Zeringue for their ideas

on the usage cold and pump induced temperature zones as a way to mitigate SBS.


henry lj et al 2010 'enhancement of output power from narrow linewidth amplifiers' optics...

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